The present invention is directed to improvements in the flash pyrolysis of carbonaceous material.
Fluid fossil fuels, such as oil and natural gas, are becoming scarce as these fuels are consumed by a world whose population is continually growing. As a consequence, considerable attention is being directed toward pyrolyzing coal and other similar solid carbonaceous materials to useful liquid and gaseous hydrocarbon products. Pyrolysis processes vary widely and include transport flash pyrolysis where pyrolysis occurs under turbulent flow conditions.
A problem exists in maximizing the yield of liquid hydrocarbons in molecular weight ranges desirable for conversion to useful end products.
Pyrolysis of coal and similar solid carbonaceous materials can produce a heavy viscous tar liquid. The tar liquid produced can be semi-solid or even solid and can have a very low hydrogen content. For example, the hydrogen-to-carbon ratio of tar liquids produced by pyrolysis of coal can typically be about 1.0.
In the past, in order to produce a marketable product, tar liquids which have been produced by pyrolysis have been hydrogenated by gaseous hydrogen to increase the hydrogen content and to remove some of the hetero atoms. Generally, high pressure gaseous hydrogen and catalysts in the sulfide form of groups VIB and VIII metals impregnated on porous solid support have been used during such hydrogenation processes. In the conventional hydrogenation of viscous tar liquids, the gaseous hydrogen consumption is very high, ranging from about 2500 to about 6000 standard cubic feet (SCF) of hydrogen per barrel of coal tar. Additionally, during conventional hydrogenation processes, the catalyst life is typically low and there is not believed to be a catalyst with proven life of more than about 200 hours of continuous on-stream operation. Generally, the high pressures and temperatures required, i.e., greater than about 2500 psig and about 600.degree. F., make hydrogenation of coal tar economically unattractive.
It is believed that the initial step in pyrolysis of coal is the thermal generation of hydrocarbon free radicals via homolytic bond scission of the coal. These hydrocarbon free radicals can be terminated by hydrogen to produce tar liquids and gas products, or they can combine with each other to produce undesirable heavy molecules such as heavy viscous tars having a boiling point above the boiling point of desirable middle distillate tar liquids. Ultimately, the hydrocarbon free radicals can continue to grow or combine with a carbon site to form char or coke.
A technique that has been used in the past, in addition to hydrogenation of high molecular weight tar liquids produced by pyrolysis, is to upgrade the tar liquids by the addition of gaseous hydrogen to the pyrolysis reactor. By hydrogenating volatilized hydrocarbons in a pyrolysis reaction zone using hydrogen gas, the value of the volatilized hydrocarbons is increased by the removal of the sulfur and nitrogen as hydrogen sulfide and ammonia. Vapor phase hydrogenation in the pyrolysis reactor also reduces the viscosity and lowers the average boiling point of the volatilized hydrocarbons by terminating some hydrocarbon free radicals before they can polymerize to form heavy high molecular weight tar liquids.
Processes involving hydrogenation are disclosed in U.S. Pat. Nos. 4,162,959 and 4,166,786. These patents disclose processes wherein coal, hot carbon-containing residue, and hydrogen gas are combined in a transport flash pyrolysis reaction zone where the coal is pyrolyzed and the pyrolysis products are simultaneously hydrogenated.
The effectiveness of hydrogen gas in terminating hydrocarbon free radicals and in hydrogenation of volatilized hydrocarbons has been found to be directly related to the hydrogen partial pressure in the reactor. The pyrolysis reaction zone of a pyrolysis reactor is preferably operated at pressures slightly greater than ambient, although pressures up to about 10,000 psig may also be used. An increase in pressure increases the hydrogen partial pressure in the pyrolysis zone and thus the effectiveness of the hydrogen in terminating free radicals and in hydrogenation of the volatilized hydrocarbons. Unfortunately, the use of high pressures increases the cost of equipment required and the total cost of the overall operation of pyrolysis. Generally, the preferred operating pressure of the pyrolysis zone, from an economical point of view, is from about 1 to about 1,000 psig, and preferably in the lower range of such pressures. The effective partial pressure of hydrogen at these pressures, however, is low and as a consequence the degree of free radical termination is less than desired.
It is known the polymerization and cracking of tar takes place rapidly at higher temperatures. Generally, vapors from pyrolysis have been condensed using either direct or indirect cooling to minimize the occurrence of secondary reactions involving combination of lighter hydrocarbon molecules into the heavier, less desirable molecules. Condensation by rapid cooling has had some effect on preventing tar from cracking, but is not completely satisfactory in preventing tar liquids from polymerizing by free radical recombination.
Processes in which pyrolytic vapors from the pyrolysis of coal are quenched with a quench fluid are described in U.S. Pat. Nos. 4,225,415 and 4,085,030.
A pyrolysis process is, therefore, desired which substantially eliminates secondary reactions in pyrolysis products and hydrogenates the pyrolysis products using less severe operating conditions, thereby economically enhancing the yield of lower molecular weight coal-derived liquids from the process.